In many laboratory settings, it is often desired to convert liquid samples into aerosols prior to chemical analysis with a spectrometer or other analytical instrumentation. Such process is often performed by use of self-aspirating nebulizers. Self-aspirating nebulizers are advantageous in many regards the majority of which are associated with the elimination of a pumping system. First, the elimination of the use of a pumping system eliminates contamination associated with the use of such system. Further, the internal volume associated with the use of a pumping system often causing rinsing as well as sample loading time to be increased is eliminated. The problems related to pumping system contamination and internal volume are extremely significant when high purity samples are to be analyzed or when the volume of the sample is small, such as with biological samples, pre-concentrated samples or radioactive samples. In addition, periodic noise introduced into the analytical signal which adversely affects the detection limits and precision of the analytical measurement caused by the periodic motion of the pumping system is removed with the elimination of the pumping system.
Although self-aspirating nebulizers presently known in the art have greatly increased the ease of converting liquid samples to aerosols prior to chemical analysis, such instruments are still disadvantageous in many regards. One of the disadvantages of current self-aspirating nebulizers is flow stoppage caused by the presence of a plurality of gas bubbles or liquid/gas segments present in an uptake capillary. For example, each bubble causes a resistance to flow. If the sum resistance to flow is greater than the suction of the self-aspirating nebulizer, flow of liquid will cease. Bubble formation is a significant concern when utilizing conventional self-aspirating nebulizers with automated sampling systems. For instance, upon the leaving of a sample probe from a sample vessel containing liquid sample (e.g. a biological sample or viscous sample) a portion of the sample liquid adheres to the outer surface of the sample probe forming a film of liquid. As the sample probe travels to another location of the automatic sampling device, a portion of the adhering liquid film flows down the sample capillary and moves over the entrance to the sample capillary, where a segment of the film is taken up, causing the formation of a bubble in the sample capillary. The liquid film may then flow down across the opening of the sample capillary again, forming additional bubbles. Such phenomenon is illustrated in FIG. 3. Approximately ten to fifty bubbles may be introduced into a sample capillary which is sufficient to prevent the self-aspirating nebulizer from operating. User intervention is required to remove such bubbles and to return the self-aspirating nebulizer to operable condition. The need of periodic user intervention removes one of the major benefits associated with automated sampling devices whereby such systems may no longer be run unattended for the possibility of bubble formation leading to self-aspirating nebulizer inoperability.
To overcome flow stoppage, a user may employ a pumped nebulizer whereby a pump can generate several atmospheres of pressure to drive a sample into a nebulizer even in the presence of bubbles or other restrictions. As such, a pumped nebulizer typically produces more reliable (although more contaminated and larger sample volumes) sample flow into the analyzing instrument when compared to conventional self-aspirating nebulizers.
An additional disadvantage associated with prior art self-aspirating nebulizers is the inability to rinse samples at high speeds between analyses. For instance, it is often desired to deliver a higher flow of rinsing liquid between samples to faster effect complete elimination of the previous sample from the instrument prior to the analysis of a subsequent sample. Conventional self-aspirating nebulizers are limited to aspirating fluid at one rate or at very narrow ranges of flow rates. By comparison, a pumping system may operate at a variety of flow rates whereby flow rate is increased by simply increasing pump speed which, in turn, creates a higher pressure in the rinse solution stream driving the rinse solution to the nebulizer at a higher speed.
Thus, a user is presently required to choose between a pumped nebulizer which introduces contamination, pulsation, and additional volume into samples or employ a self-aspirating nebulizer which is plagued by unreliable sample flow and limitations on sample rinsing which requires periodic user intervention obviating the use of an automatic sampling system with unattended runs.
Therefore, it would be desirable to design a system which allowed a self-aspirating nebulizer to be utilized with an automated sampling device whereby the present limitations associated with conventional self-aspirating nebulizer of flow stoppage and inability to significantly alter flow rates were eliminated.